The present invention relates to interference suppression systems and more particularly to techniques for improving cancellation in a side-lobe canceller system.
Generally, in signal receiving systems such as radar systems, signals received in the side-lobes of the radar antenna interfere with the isolation of desired signals received in the main lobe of the antenna. Although attempts have been made to eliminate the side-lobe interference by employing various side-lobe canceller systems, the same have only met with limited success in different situations.
One such system, as exampled by U.S. Pat. No. 3,202,990, employs a plurality of adaptive canceller loops to operate on offset auxiliary signals to subtract interference received by the main radar antenna. While such conventional systems have been relatively successful in achieving good cancellation, it has been found that the use of the parallel loops leads to a configuration that tends to be unstable due to loop interaction when high gain is used and thereby limits the maximum cancellation achievable.
In other prior known techiques, baseband adaptive loops use quadrature hybrid circuits to separate the auxiliary signal into I and Q components. I and Q in this case refer to the interfering signal phase shifted by zero and ninety degrees respectively, independent of frequency. Each of the components is coupled to form its own adaptive loop coupled in parallel with the other, and coupled to subtract the auxiliary signal from the signal received in the primary antenna to thereby reduce interference. The two loops in parallel make the circuit tolerant of phase and amplitude errors from the quadrature hybrid, but introduce the possibility of instability due to loop to loop oscillation at high gain. As before, such a limitation on gain limits the degree to which interference can be removed from the primary radar signal.
Accordingly, the present invention has been developed to overcome the shortcomings of the above known and similar techniques and to provide an interference cancelling system with improved cancellation performance.